Using pulse width modulation scheme for LCD back lighting an app note from Hantronix. Link here (PDF)
LED back lights on LCD modules are generally driven with a dc voltage through a current limiting resistor. This simple approach is perfectly acceptable for most applications. When the primary consideration is an extra bright display, the lowest possible power consumption, or a back light that can be controlled over a very wide brightness range another method is needed. The purpose of the paper is to describe this method.